It is known to provide vehicle having a diesel engine with a diesel aftertreatment system incorporating a diesel oxidation catalyst (DOC). The DOC uses oxygen in the exhaust gases to convert carbon monoxide (CO) to carbon dioxide (CO2); and to convert hydrocarbons (HC) to water (H2O) and (CO2). In order for these reactions to occur efficiently, the catalyst must be heated to above a critical operating temperature, the so-called ‘light-off’ temperature. However, with a view to improving fuel efficiency and performance, this results in a reduction to exhaust gas temperatures. This results in an increased time period until the catalyst reaches its light-off temperature. It is known to use exhaust heating strategies to reduce the light-off time, but these impact on fuel economy. An alternative approach is to use catalysts having a lower light-off temperature.
A further problem arises when the diesel engine is turbocharged to improve performance. This results in increased thermal inertia of the exhaust system upstream of the DOC along with increased heat extraction. These factors can further delay achieving the light-off temperature of the catalyst.
A further limit on operating temperature arises with urea selective catalytic reduction systems (uSCR) for aftertreatment of nitrogen oxides. A lower temperature limit must be imposed to achieve the required decomposition of urea to ammonia whilst preventing the creation of solid urea deposits. This temperature limit is typically 180° C., but this may not be achieved during certain driving conditions, for example when the vehicle is driven in a city. Under these operating conditions, the operation of the uSCR system may be restricted due to low diesel exhaust gas temperatures.
The recirculation of exhaust gases to the engine to reduce emission of nitrogen oxides, so called high-pressure exhaust gas recirculation (HP-EGR), can also present problems. The exhaust gases are recirculated from the exhaust manifold to the engine intake manifold, but hydrocarbons and particulates in the exhaust gases can cause contamination and potential durability issues within the engine.
Finally, the packaging of diesel emissions aftertreatment systems, typically including oxidation catalysts (DOC), catalysed diesel particulate filters (cDPF) and uSCR, remains a constant challenge. Current pre-turbocharger catalysts and exhaust gas recirculation catalysts tend to be limited in size due to use of conventional honeycomb structures. The catalysts therefore require additional space and thus may only operate at low speed/load conditions and require an additional post-turbocharger conventional DOC.
It is against this backdrop that the invention(s) described herein have been conceived. At least in certain embodiments, the present invention seeks to overcome or ameliorate at least some of the shortcomings associated with known aftertreatment systems.